Previous studies in this laboratory have demonstrated that during pregnancy there occurs an activation of a maternal spinal opioid system(s) that mediated characterize the specific opioid system(s) that is being modulated (type of opioid receptor and endogenous ligand that activates it) during gestation. The specific objectives are to (1) Determine the type(s) of spinal opiate receptor (mu, delta, kappa) that mediates the spinal opioid analgesia of pregnancy. (2) Determine the endogenous opioid ligand(s) (enkephalin, dynorphin, B-endorphin) that mediates the spinal analgesia of pregnancy. (3) Determine the concentration profile of enkephalin, dynorphin, B-endorphin and total opioid activity in spinal cord superfusate of pregnant (day 20) and non- pregnant rats. In addition to the above, experiments will be conducted to determine the facet(s) of pregnancy (changes in peripheral hormone concentration, increased pressure against the uterine cervics, etc.) that is responsible for the increase in spinal opioid activity during gestation. The specific objectives will be to (4) Determine if the analgesia associated with pseudopregnancy involves a spinal opioid component as does the analgesia that is associated with actual pregnancy, and (5) Determine the effect of increased force applied to the uterine cervix on pain thresholds on non-pregnant female and pseudopregnant rats. This study will indicate if the increase pressure against the uterine cervix that develops naturally during pregnancy and labor might be at least one of the internal stimuli that results in the activation of maternal opioid system(s) and the concomitant elevation in pain threshold.